208 GROWTH HORMONES IN PLANTS 



tion method) . These results make it highly probable that the pres- 

 ence of sugar in the agar promotes auxin formation in the root. 

 Boysen Jensen and others have determined the amount of 

 growth substance contained in the various root zones and found 

 that the amount decreases gradually from the tip backward, 

 being confined mainly to the apical 6 mm. It can be assumed 

 on the same grounds as previously mentioned in the chapter on 

 phototropism that no special geotropic hormone is formed in the 



root tip. 



Transverse Distribution of Growth Substance.^ — Boysen Jensen 

 (1933c) showed that more growth substance is given off into a 

 dextrose-agar block from the under half of a geotropically 

 stimulated root tip than from the upper half. The experiments 

 were carried out in the following way: After seedlings of Vicia 

 Faba had been in a horizontal position for Z}4 to 4 hours at a tem- 

 perature of 19°C, the root tips (about 2 to 4 mm.) were cut off with 

 a razor. Blocks of dextrose agar were placed one above the other 

 upon a carrier at a distance of 0.55 mm. from each other (Fig. 2). 

 The root tips were then placed upon these agar blocks in such a 

 way that the upper half rested upon one block and the lower half 

 upon the other block. The root tips and agar were then placed 

 in a saturated atmosphere for 2 to 5 hours after which the tips 

 were removed from the agar. With the help of the marks which 

 are left by the tips upon the agar blocks, it was possible to select 

 blocks on which the tips had been precisely placed. These were 

 moistened with an alcohol-citric acid solution and kept overnight 

 at a low temperature. The following day they were placed 

 unilaterally upon decapitated Avena coleoptiles, and the resulting 

 curvatures were measured. It was found that the blocks that 

 had received growth substance from the under halves of the root 

 tip produced greater curvature than those treated with the upper 

 halves. It may be concluded that more growth substance is 

 given off from the under than from the upper side of a geotropi- 

 cally stimulated root tip. Further experiments with the root 

 tips kept in a horizontal and in a vertical position while the 

 growth substance was extracted yielded a difference between the 

 d values in the former instance amounting to 0.40 mm. (27 

 plants) and, in the second case, 0.32 mm. (37 plants). 



Although the unequal distribution of growth substance in the 

 root tip, due to gravity, could come about in various ways, it may 



